Manheim Township High School sent students home early Friday after a bullet was found on a classroom floor.
Township police said a student found the small-caliber round and turned it over to a teacher.
The building was placed on administrative lockdown while authorities searched students who were in the classroom and the surrounding area, but nothing else was found.
Administrators began dismissing students around 12:30 p.m. because they could not determine who brought the bullet into the school.
All students were searched at dismissal.
Other schools in the district operated as scheduled.